In order to maintain an exact centering for precision turning and cylindrical grinding one does not clamp the workpiece in the chuck of the lathe but positions it between centres which determine its rotation axis. Because the friction between the centres and the workpiece is insufficient to rotate the workpiece, the torque needed for machining the workpiece is not transmitted directly from the lathe to the workpiece, but indirectly by means of an attachment mounted on the outer surface of the workpiece and usually fixed by force-locking, e.g. through the action of a radial screw. This attachment comprises simple means which allow a certain amount of eccentricity between the axis of the mechanism that provides the torque necessary for machining and the rotation axis of the workpiece without exerting noticeable radial forces on the workpiece.
Traditional attachments for this purpose usually consist in a simple clamp, sometimes called a "drive dog", fixed to the workpiece by means of one or several radially oriented screws. A finger that rotates with the spindle of the lathe pushes against a stop of the clamping attachment situated at a certain distance from its rotation axis and urges it around, thereby transmitting the rotation of the lathe to the clamping attachment, and hence to the workpiece. Provided the finger can easily glide radially hither and tither on the stop, it will transmit only a torque and practically no radial forces to the workpiece, even when the rotation axis of the attachment does not coincide exactly with that of the spindle. However, this simple device is unsatisfactory for an automated production cycle because of the time required for mounting and dismounting it. Thus, in many applications the clamping attachment has been replaced by a compensating chuck with a floating axis the jaws of which can rotate with an eccentricity of up to a few millimeters with respect to its driving mechanism. The jaws can thus grip the outer surface of a workpiece and drive it even if its surface is not concentric with the rotation axis determined by the centres. But the comparatively large weight of such a chuck generates out of balance forces which must be compensated at high rotation speeds, and this considerably increases the manufacturing costs of compensating chucks. Further, practical considerations make it well-nigh necessary to transmit the relatively large forces needed for gripping the workpiece, or alternatively for pre-tensioning the springs that provide the clamping forces, first axially and then to divert them--either by mechanical or by hydraulic means--into the radial directions required for actuating the jaws of the chuck. This further increases the dimensions, the weight, and the costs of compensating chucks.
Machine tools often comprise a standard chuck with three jaws (with concentric action) and it was hitherto necessary to replace it with a special chuck if the workpiece was to be held between centres. Therefore, it is desirable to devise a torque transmission that exhibits as far as possible the light weight and simplicity of a drive dog attachment, but can nevertheless be used on half or fully automatic machine tools without requiring the complexity of a compensating chuck. To achieve this goal the invention is defined as recited in the claims.
The invention allows one to use a conventional chuck, together with a light and cheap clamping attachment the spring of which can be pre-tensioned either by closing or by opening the chuck. The invention thus provides a simple way to implement an automatic production cycle. In preferred embodiments, the external pre-tensioning forces are aligned with the forces exerted by the springs of the clamping attachment, which makes for an especially compact design and avoids friction losses due to intermediate mechanisms for diverting the pre-tensioning forces in the clamping attachments, or for producing a parallel shift of these forces.